1971 IMO Problems

Problems of the 13th IMO 1971 in Czechoslovakia.

Problem 1

Prove that the following assertion is true for $n = 3$ and $n = 5$ , and that it is false for every other natural number $n > 2$ :

If $a_1, a_2, \cdots, a_n$ are arbitrary real numbers, then $(a_1 - a_2)(a_1 - a_3) \cdots (a_1 - a_n) + (a_2 - a_1)(a_2 - a_3) \cdots (a_2 - a_n) \\ + \cdots + (a_n - a_1)(a_n - a_2) \cdots (a_n - a_{n - 1}) \geq 0$

Solution

Problem 2

Consider a convex polyhedron $P_1$ with nine vertices $A_1, A_2, \cdots, A_9$ ; let $P_i$ be the polyhedron obtained from $P_1$ by a translation that moves vertex $A_1$ to $A_i (i = 2, 3, \cdots, 9)$ . Prove that at least two of the polyhedra $P_1, P_2, \cdots, P_9$ have an interior point in common.

Solution

Problem 3

Prove that the set of integers of the form $2^k - 3 (k = 2, 3, \cdots)$ contains an infinite subset in which every two members are relatively prime.

Solution

Problem 4

All the faces of tetrahedron $ABCD$ are acute-angled triangles. We consider all closed polygonal paths of the form $XYZTX$ defined as follows: $X$ is a point on edge $AB$ distinct from $A$ and $B$ ; similarly, $Y, Z, T$ are interior points of edges $BC, CD, DA$ , respectively. Prove:

(a) If $\angle DAB + \angle BCD \neq \angle CDA + \angle ABC$ , then among the polygonal paths, there is none of minimal length.

(b) If $\angle DAB + \angle BCD = \angle CDA + \angle ABC$ , then there are infinitely many shortest polygonal paths, their common length being $2AC \sin(\alpha / 2)$ , where $\alpha = \angle BAC + \angle CAD + \angle DAB$ .

Solution

Problem 5

Prove that for every natural number $m$ , there exists a finite set $S$ of points in a plane with the following property: For every point $A$ in $S$ , there are exactly $m$ points in $S$ which are at unit distance from $A$ .

Solution

Problem 6

Let $A = (a_{ij})(i, j = 1, 2, \cdots, n)$ be a square matrix whose elements are non-negative integers. Suppose that whenever an element $a_{ij} = 0$ , the sum of the elements in the $i$ th row and the $j$ th column is $\geq n$ . Prove that the sum of all the elements of the matrix is $\geq n^2 / 2$ .

Solution

1971 IMO (Problems) • Resources
Preceded by 1970 IMO	1 • 2 • 3 • 4 • 5 • 6	Followed by 1972 IMO
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1971 IMO Problems

Contents

Problem 1

Problem 2

Problem 3

Problem 4

Problem 5

Problem 6

See Also